Lignocellulose is the main renewable resource with several applications, including feedstock for bulk chemicals. The main components of lignocellulose are cellulose, hemicellulose and lignin. The cellulose fraction comprises polymers of the hexose sugar, glucose. The hemicellulose fraction is mainly comprised of polymers of the pentose sugars xylose, and arabinose, and the hexose sugars mannose, glucose and galactose. These are abundant in the lignocellulosic biomass of soft woods, whereas xylose occurs in both soft and hard woods.
There are numerous examples for efforts to utilize lignocelluloses more effectively by microorganisms and to extend the substrate utilization range of microorganisms towards low-cost and abundant carbon sources. For example, attempts have been made to broaden the utilization of unfermentable pentoses like xylose and arabinose occurring in lignocellulosic biomass in many microorganisms. Many of these investigations aimed at the extension of carbohydrate metabolism in ethanol producers, such as Zymomonas mobilis and Saccharomyces cerevisiae for ethanol fermentation bioprocess. WO 2009/08190, for example, describes genetical modification of ethanol producing bacterium belonging to the genus Zymomonas to utilize glucose, mannose, and xylose.
Bacteria belonging to genus Ralstonia and Cupriavidus are chemolithoautotrophs which are capable of accumulating large amounts of polyhydroxyalkanoates (PHAs) from renewable resources or from carbon dioxide. Strains of the genus Ralstonia and Cupriavidus, in particular, R. eutropha (new name Cupriavidus necator) have been shown to possess large biotechnological potential as these strains can be cultivated to high cell densities in large scale (Schlegel et al. 1961) and is suitable for large-scale industrial bioprocesses. Unfortunately, Cupriavidus and Ralstonia have a very narrow carbohydrate substrate range limited to fructose, N-acetylglucosamine and gluconate. Attempts to broaden the substrate utilization range of R. euthropha H16 have been described in the prior art (Pries et al. 1990, Schlegel and Gottschalk, 1965). A spontaneous glucose-utilizing mutant of R. euthropha H16 which presumably transported this sugar into the cell by a passive transporter has been reported to be isolated (Schlegel and Gottschalk 1965, König, et al. 1969). Furthermore, Pries et al. 1990 have reported of the introduction of β-galactosidase gene and the gal operon from E. coli into R. eutropha strain H16 and utilization of lactose and galactose by this bacterium.
Further, Buchholz et al. 1994 have described the construction of a genomic library of Pseudomonas saccharophila and its transfer to Alcaligenes eutrophus (Ralstonia eutropha). An insert containing xyl genes encoding xylose isomerase and xylulokinase made the host A. eutrophus (pGN3) to grow on xylose.
In spite of the previous attempts to extend the substrate utilization range of certain microorganisms towards low-cost and abundant carbon sources, there is still a need for new methods and microorganisms for more efficient utilization of renewable resources, in particular lignocelluloses.